Chapter 6, COMPUTER OPERATION
6-36
6.5 Programming Examples
6.5.01 Introduction
This section gives some examples of the commands that need to be sent to the lock-
in amplifier for typical experimental situations.
6.5.02 Basic Signal Recovery
In a typical simple experiment, the computer is used to set the instrument controls
and then to record the chosen outputs, perhaps as a function of time. At sampling
rates of up to a few points per second, there is no need to use the internal curve
buffer. The commands to achieve this would therefore be similar to the following
sequence:
IE 2 Set reference to external front panel input
VMODE 1 Single-ended voltage input mode
AUTOMATIC 1 AC Gain control automatic
FLOAT 1 Float input connector shell using 1 k to ground
LF 0 0 Turn off line frequency rejection filter
ASM Auto-Measure (assumes reference frequency > 1 Hz)
TC 13 Set time constant to 200 ms, since previous ASM changed it
Then the outputs could be read as follows:
X. Reads X channel output in volts
Y. Reads Y channel output in volts
MAG. Reads Magnitude in volts
PHA. Reads Phase in degrees
FRQ. Reads reference frequency in hertz
The controlling program would send a new output command each time a new reading
were required. Note that when using an output filter slope of 12 dB/octave a good
"rule of thumb" is to wait for a period of four time-constants after the input signal
has changed before recording a new value. Hence in a scanning type experiment, the
program should issue the commands to whatever equipment causes the input signal
to the lock-in amplifier to change, wait for four time-constants, and then record the
required output.
6.5.03 Frequency Response Measurement
In this example, the lock-in amplifier's internal oscillator output signal is fed via the
filter stage under test back to the instrument's signal input. The oscillator frequency
is stepped between a lower and an upper frequency and the signal magnitude and
phase recorded. At sampling rates of up to a few points per second, there is no need
to use the internal curve buffer or oscillator frequency sweep generator. The
commands to achieve this would therefore be similar to the following sequence:
IE 0 Set reference mode to internal
VMODE 1 Single-ended voltage input mode
AUTOMATIC 1 AC Gain control automatic
FLOAT 1 Float input connector shell using 1 k to ground
LF 0 Turn off line frequency rejection filter
OA. 1.0E0 Set oscillator amplitude to 1.0 V rms
OF. 100.0 Set oscillator frequency to 100 Hz (starting frequency)
SEN 27 Set sensitivity to 1 V full-scale
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